In this theory I’ll talk about a man that probably discovered Raftel or went quite close to it and that with his adventures inspired many one of them the only known Pirate King Gol D. Roger. This man is MontBlanc Noland.
Theory
Noland lived 400 years ago in the One Piece Timeline and was an admiral of the Lvneel Kingdom. He made multiple expedition in the Grand Line and even explored the New World. For that time coming back just form one expedition from the Grand Line unscratched physically or mentally was impossible. Yet this man went multiple times and come back with all his crew. Noland was insanely strong, he was able to single-handedly kill a Sea King underwater.
He was even able to kill Kashigami, which was the same size of his grandchild Nola, with a single slash of his sword. Something that Zoro and Wiper together couldn’t mange to do with Nola.
So he was strong, his strength could be enough in the current time to put him on a suitable Pirate King pretend like the Supernovas are or maybe even a Yonko why not 
So why “first Pirate King”?
You see this is the crazy part in this theory aahah for one to be declared Pirate King he must arrive on Raftel. Before Roger, this island was a legend and it’s still in the current time. I believe that Noland might arrived on Raftel or very near to it, it’s known that when he came back from his journeys he usually talked about his crazy adventures.
But after he failed to reach Jaya for the second time with his king he was sentence to death and his legacy all his adventures where considered a lie because the king said so. In the story about Noland the king was portrayed as valorous warrior and Noland as a hopeless fool.
So if he ever said something about Raftel people would think at it as a stupid old legend, even after Roger found it many still believe it’s a legend.
This could be crazy but we found out that that Noland arrived on Dressrosa and fought the evil people that where after the tontattas.
Where ever an evil “god” or evil was, Noland had arrived on that island before and left his mark. When Luffy arrived at Skypea we had God Enel, in Noland times there was Kashigami, he and Luffy defeated them. On Dressrosa we had Doffy that had captured the tontattas, Noland 400 years ago fought people with the same intentions. So I believe he left his mark on Raftel as well where Luffy would have to fight BlackBeard. I can not understand otherwise why would Oda show us Noland again on Dressrosa where there was a similar plot to that on Skypiea.
He inspired Roger 
I know this is crazy too aahah
but his story is very similar to that of Francisco de Orellana a spanish conquistador that claimed that he had found El Dorado the city of gold. Many thought at it as a stupid lie, but there where others that believed in it and went after it and many are still searching for it today. So I believe that there where people that went on searching in One Piece as well. Further more Noland’s family name is MontBlanc, you see Mont Blanc is considered to be the one that made modern Alpinism or mountain climbing. So Noland’s inspiration(Francisco de Orellana) and family name gave birth to new adventures and because of this I believe his story and adventures made many set sail, because they had nothing to lose.
Imagine a child reading Noland’s story he would be intrigued by it and I believe Roger was inspired as well he was a child. I believe I have the proof I showed you an imagine at the beginning, this one:
Could you see the kid with the straw hat? I believe the boy in the image is a hint to that, but it isn’t Roger because the boy in the image is form 400 years ago, so he might be a previous possessor of the Straw Hat. So if Noland ever talked about Raftel or just his adventures ignited a little sparkle in child Roger that would want to explore as well the sea.
*Theory by MasterD
